Air intercept computer



April 30, 1963 F. E. SMITH 3,088,108

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INVENTOR. FREDERICK E. SMITH nfroausvs 3,088,108 Patented Apr. 30, 1963 [ice 3,088,108 AIR iNTERCEPT COMPUTER Frederick E. Smith, Southampton, Pa., assignor to the United States of America as represented by the Secretary of the Navy Filed Feb. 20, 1958, Ser. N 716,518 29 Claims. (Cl. 3437) [Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes Without the payment of any royalties thereon or therefor.

The present invention relates to an intercept computer and more particularly to an intercept computer in which the plan position data of an early warning radar is used for computing the heading and time required for an interceptor aircraft to intercept a moving target at a remote unique point in space.

Various methods have been devised for computing a collision course approach and the flight time required for an interceptor aircraft to successfully intercept a moving target at a remote point in space. The problem is capable of solution by means of analog computers which have been previously programmed. Hand calculators using the display data of a plan position indicator (PPI) in an early warning radar have also been employed. However, the devices of the prior art generally possess various inherent limitations which preclude a feasible solution of the problem, especially in view of the supersonic speeds of contemporary aircraft. Further, because of space and weight considerations, analog computers, in general, do not conveniently adapt for airborne use, in addition to the fact that they are susceptible to operational dilliculties peculiar to these instrumentalities. Hand calculators lack accuracy and fail to have a facility commensurate with the tactical requirements.

The intercept computer of the instant invention utilizes the video data presented on the screen of a PPI scope in an early warning radar and augments the presentation to include an electronically generated slewable cursor and circle which are displayed in time shared relationship with the normal radar sweeps so that the radar scope opcrator is enabled to readily compute both a collision course approach to an enemy aircraft and the time required to make the interception from the position of a friendly aircraft. The aforesaid cursor and circle are each associated with the target and interceptor video, respectively, under selective control of the operator such that an intersection therebetween yields the desired solution in accordance with the simple equation Distance=Speed Time With the aid of a selectively controllable cursor strobe or dot superposed on the cursor sweep which itself is rotatably controlled about its origin, a determination of the target course and speed is initially made from a knowledge of the immediate history of the target video, which remains momentarily visible due to screen persistence. Corterminous with this determination, the circle of radius proportional to the instantaneous product of time and a predetermined interceptor speed is superposed onto the interceptors position as viewed on the screen. Under control of the radar scope operator, a timing mechanism provides for extrapolating current target position into future time position by varying the time parameter, simultaneous with an increasing interceptor circle radius until an intersection occurs therebetween at a unique intercept point. A circle azimuth dot movable about the periphery of the circle facilitates the determination of an intercept heading. A Time-to-Intercept indicator is automatically set during this operation and continually indicates the flight time remaining for the interceptor travelling a predetermined rate of speed to intercept the target. This information is continually available during the course of an intercept problem and is relayed to the friendly interceptor aircraft by a conventional data link transmission means. Thus, a combat information center (ClC) incorporating the instant invention is capable of directing the intercept of enemy aircraft with dispatch. The instant invention has been successfully employed with a mobile C10 in conjunction with an airborne early warning radar. The presentation under such operational circumstances is both north and ground stabilized so that the conveying aircraft may be in motion, and yet stationary targets will remain in a fixed position on the face of the cathode ray tube (CRT), as is understood.

An object of the present invention is to locate the point in space where the flight paths of two aircraft may be made to intersect by having control over the flight of one of the aircraft.

Another object is to provide an intercept computer in which the heading and the flight time required to intercept a target from the position in space of an interceptor aircraft may be readily determined in a finite time interval consonant with tactical requirements.

Still another object is to provide an intercept computer in which the heading and the time required to intercept a target at a remote point in space from the position of an interceptor aircraft is uniquely determined by the point at which a target strobe superposed on a slewable cursor sweep mutually intersects a circle of radius proportional to the instantaneous product of time and a predetermined interceptor speed.

A still further object of the present invention is the provision of an intercept computer in which a collision course approach and the time required to intercept a moving target from the position of an interceptor aircraft is computed in accordance with the equation Distance=Speed Time such that the respective distances traversed in equal intervals of future time by the interceptor aircraft and the target is predicated upon the known speed of the friendly interceptor aircraft and the speed of the target determined by the coincident travel of target video with an electronically generated strobe superposed on a cursor sweep angularly disposed to coincide with the target heading.

A final obicct of the present invention is to provide in an intercept computer a timing mechanism for rendering a chronometric rate of rotation required in the multiplication process, wherein the current time established by the aforesaid chronometric rotation may be manually projected into future time to effect determination of a target intercept point.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIG. I is a plan position view of the electronically generated cursor and circle representatively displayed in relative orientation on the screen of a PPI scope,

FIG. 2 is a simplified diagram of the speed and time multiplication system incorporated in the instant invention,

FIGS. 3a, 3b, and 3c is a composite functional block diagram of a preferred embodiment of the instant invention,

FIG. 4a through FIG. 4k is a composite detailed electrical schematic drawing of the invention,

FIG. 5 is a timing chart particularly showing the relative amplitude and timing relationships of the more pertinent wave forms found at various points in the electrical circuits of the inventive intercept computer,

FIG. 6 is a diagrammatic view in isometric form of the timing mechanism of the instant invention,

FIG. 7 is a diagrammatic view of the portion of the timing mechanism more pertinently associated with the running mode of operation, and

FIG. 8 is a diagrammatic view of the portion of the timing mechanism, more pertinently associated with the reset mode of operation.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a representative PPI display of an early warning radar set which incorporates the inventive intercept computer. Cursor 11 and circle 12 are portrayed typically disposed wherein the size of the circle has been expanded during the advance time mode of operation of the timing mechanism to intersect the cursor at the intercept point P. This mode of operation will be subsequently described with greater particularity with relation to FIG. 6. Cursor 11 is illustrated in prolongation with successive plots of target video T, thus establishing coincidence of the cursor with the direction of flight of the target, the speed of which is determined by coincident travel of the target viedo T with an electronically generated cursor dot or strobe S movable along the cursor and shown superposed onto the intercept point P in the view of FIG. 1. The interceptor circle 12 is portrayed with its center at the friendly interceptor aircraft video I. Since its radius is directly proportional to the product of the interceptor speed and time, the mutual intersection of cursor sweep 11 and expanded circle 12 coincident with projected cursor dot S at point P yields the intercept heading a, measured from a north reference as indicated. To facilitate a measurement of this angle, a circle azimuth dot D is provided. Upon retracting to current time operation from advance time operation, a time-to-intercept indicator is automatically set, and thereafter indicates continuously the flight time remaining for the interceptor travelling a predetermined rate of speed to intercept the target.

FIG. 2 portrays a simplified showing of the speed and time multiplication system incorporated in the instant invention. The specific computation hereinbefore denoted involves only the multiplication of speed and time to produce distance. The basic technique for performing a solution of this equation is best delineated with respect to the simplified showing provided in this view. Accordingly, there is schematically illustrated in FIG. 2 a timing mechanism 90 comprising a motor 16 having constant speed characteristics serving in the instant invention a chronometric function, a dilferential 17 including a time modifier control 19, a time dial 18 utilized as a time-tointercept indicator, and time potentiometers R13 and R14. The elements herein set forth are mechanically coupled, as indicated by the dotted lines, and will be discussed subsequently in greater particularity with respect to FIGS. 6, 7 and 8. The chronometric rotation of motor 16 will be seen in FIG. 2 to be transmitted through differential 17 and time dial 18, to the time potentiometer housings or resistive portions proper of R13 and R14, which thereby rotate at a constant rate of speed relative to the respective wipers. Time modifier control 19 is a manual input to differential unit 17 operable to modify the relative positions of the potentiometer wipers with respect to the respective housings or resistive portions of the poten tiometers. The setting of target speed control potentiometer R80, a function of target speed as determined by progression of target video on the face of the PPI scope is inserted by manual adjustments as necessary to maintain cursor dot S coincident with the target T as it advances along the cursor. Similarly, the setting of the interceptor speed control potentiometer R90 is manually adjusted to the known speed capability of the interceptor aircraft.

Apropos of the structure depicted in FIG. 2, and understanding of the multiplication process performed in the inventive intercept computer may be had by a consideration of time potentiometer R14 with target speed control potentiometer R80. Assuming a constant D.C. voltage X to be impressed across potentiometer R14, the wiper thereon linearly fractionates this voltage in direct proportion to the elapsed time by virtue of the constant speed drive transmitted from chronometric motor 16. Since coefficient K has a range between 0 and l, the potential between the wiper and ground is K X, an analog voltage which thus relates to the elapsed time in the computer. With the aid of time modifier control 19, time may be projected into the future, and accordingly, any future time interval may be expressed as a proportionate voltage. The voltage K X will be observed to be the reference voltage for target speed control potentiometer R80. Consequently, K X is further modified by a second coelficient K which has a similar range and is set according to the target speed. Hence, the analog voltage K K X available at the potentiometer 21 relates to target distance, already traversed or anticipated during a selected time interval. This voltage is then applied to the circuits of the cursor dot generator.

Inasmuch as the function and operation of potentiometers R13 and R90 are similar to that of potentiometcrs R14 and R80 with regard to the multiplication process, the explanation set forth above is deemed sufficient. A minor but significant variation exists in that the reference voltage Y is sinusoidal in this instance to facilitate the electronic generation of a circular pattern. The magnitude of analog voltage C C Y relates to interceptor distance, traversed or anticipated in the same selected time interval. This voltage is thence supplied to appropriate circuits for the generation of the interceptor circle.

Referring next to FIG. 3a, FIG. 3b, and FIG, 30, there is shown in a consecutive arrangement of these views, a functional block diagram of a preferred embodiment of the instant invention. As hereinbefore mentioned, the electronically generated cursor and circle are presented in time shared relation with the radar PPI sweeps. The early warning radar set which incorporates the instant invention has a pulse repetition rate (PRF) of 300 per second or thereabouts, and in accordance with this design criterion, the instant invention provides for every tenth sweep to be alternatively displayed on the PPI screen either as a cursor sweep or as a circular pattern. Therefore, the recurrence rate at which either the cursor or circle is presented occurs at a 15 cps. rate, which is compatible for comfortable viewing without evidence of flicker. The sweep generating means including numerous components represented by various blocks in FIGS. 35 and 3c is arranged to accomplish this objective in the manner described below. A transmitter pulse amplifier 24 is depicted in FIG. 3b, the twofold purpose of which is to amplify a positive radar trigger appearing at input tcrrninal 23 and invert the phase of the signal in order that it may be of a desired negative polarity required in subsequent applications thereof. Transmitter puise cathode follower 25 functions to provide isolation in addition to supplying a low impedance trigger of approximately 80 volts to the multivibrator gate 26 and phantastron cathode follower 77. The sweep limiter and charging clamp 27 is interrelated in its actions with multivibrator gate 26 to concurrently produce a pair of bistable output voltages of a substantially square wave character and the requisite sawtooth waveforms necessary for effecting sweep lengths corresponding to 2G, 50, I00 and 200 mile ranges. The square wave voltages of approximately 80 volts in amplitude are opposite phase and occur in response to the triggers appearing at terminal 23. The respective character of these voltage waveforms in relative amplitude and phase relation may be observed in the tinting chart of FIG. 5, waveforms B and D being typical outputs of multivibrator gate 26 for a selected range sweep length.

The sawtooth amplifier 2S amplifies the sawtooth voltage which is thus applied to a synchro driver 29. The latter stage is a power cathode follower which feed into the rotor windings of synchro resolvers 31 and 32, the respective stator windings of which are disposed ninety electrical degrees apart. The purpose of these resolvers is to convert the applied sawtooth voltage into NS and EW sawtooth components, which are proportional to the angular displacement of the respective rotors. The rotor of resolver 31 is indicated by the dotted line notation to be mechanically coupled with the servo followup drive of the antenna system, and its rotation is thereby in synchronism with the radar antenna. The rotor of resolver 32 is angularly displaced under manual control of the operator by means of cursor bearing control 33. The sawtooth components from both resolvers are applied to the PPI and cursor switch stages 34, 35, 36 and 37, comprising dual triode type envelopes which are responsive to appropriate gating pulses, permitting selective application of the PPI and cursor sawtooth voltage components to the N-S and E-W push-pull deflection amplifiers 38 and 39, in accordance with the successive presentation of nine PPI sweeps for each circle or cursor sweep. With respect to either the circle or cursor presentation, N-S and E-W components of clamping voltage maintained at the level of the slewing input signal are supplied to amplifiers 38 and 39. Provision is made to clamp the normal PPI sweeps at ground potential, preventing the location of the start of the trace from shifting due to rotation of the sweep. It will be observed in this respect that the sweep signal outputs of the switch stages are in common connection with the respective outputs of the PPI and cursor gated clamp stages 41, 42, 43 and 44. Thus, in general it may be presently noted that a composite sweep signal which includes a DC. slewing component is supplied to the push-pull deflection amplifiers, the output load of which contains an inductive yoke L907 having deflection coils which are quadrantally disposed about the neck of the cathode ray tube 45. Coherent unblanking or lowering of the cutoff bias on the CRT is provided by a brilliance and blanking circuit 48. Hence, as summarily described above with respect to the composite functional block diagram of the instant invention, PPI and cursor sweeps are generated and selectively displayed coincidcntly with intensity modulation of CRT45.

In addition to being of transcendent importance in the generation of sweep signals, the square wave output of multivibrator gate 26 is supplied to count-down multivibrator circuit 46. As previously denoted, this waveform has a recurrence rate which is equal to the PRF of the radar triggers. It is the purpose of multivibrator 46 to fractionate or count down this recurrence rate by ignoring a certain number of differentiated waveforms, precisely nine cycles, and responding to the tenth waveform. Multivibrator 46, therefore, functions to generate a substantially asymmetrical bistable voltage at a recurrence rate of 30 c.p.s. This voltage is supplied to switch gate generator 47 wherein two square waves of opposite polarity are produced having stable limits which vary between +110 and 249 volts. The cyclic period of these specific bistable voltages is unchanged, being of the same duration as the output from count-down multivibrator 46. To better delineate from a systems point of view the plurality of gating functions served by these particular square wave signals, they have been designated the PPI and cursor side outputs, which are representatively indicated in FIG. 5 by waveforms E and F, respectively.

The brilliance and blanking circuit 48 is of a design requiring both the PPI and cursor gating signals in addition to the bistable signal of multivibrator 26 to provide time shared intensity modulation of the cathode of CRT45, and of particular note, relay driver 50 in FIG. 3a is fed the signal associated with the PPI side. The latter stage effects actuation of a pair of differentially operated high speed relays K301 and K302, at a repetition rate which is one-half the incoming signal. Therefore, the respective contacts of these relays complementally switch back and forth at a 15 cps. rate, that is to say, when the alphabetically designated contacts of K301 are in the down position shown, the lower alphabetically designated contacts of K302 are in the up position, and vice versa. Switch gate generator 47 also is a source of gating signal to the PPI and cursor switch stages 34, 35, 36 and 37, the PPI and cursor driver cathode followers 55 and 56, and video and strobe switches 52 and 53. The pertinence of these PPI and cursor gating signals will become more apparent in subsequent description of the system operation in relation to the timing chart depicted in FIG. 5.

With respect to the description set forth in connection with the representative PPI display shown in FIG. 1, the cursor strobe S and azimuth circle dot D including radar video are selectively inserted on a time shared basis into the video channel of the instant intercept computer. In FIG. 30 radar video is applied at terminal 49 and undergoes amplification in video amplifier 51 where it emerges as a positive signal voltage. A video switch 52 upon receipt of a PP! gating signal from switch gate generator 47 permits passage of the video to cathode follower 54 during generation of the radar PPI sweeps. Strobe switch 53 functions in a comparable manner to alternately in sert into the video channel positive spike voltages representing the azimuth circle dot D and cursor dot S upon being conditioned by the gating signal from the cursor side. Hence, the positive waveform presented at cathode follower 54 consists of a composite signal voltage which includes azimuth circle and cursor dot voltages and radar video. The positive signal from cathode follower 54 is appropriately clamped by clamper 60 and applied to the grid of CRT4S. Thus, in the manner herein described the video channel comprising the elements set forth above facilitates the generation of a coordinated time shared display.

The structural elements particularly portrayed in FIG. 3:: provide in general for the consummation of the multiplication processes of the instant invention, the generation of sinusoidal components for presentation of a circular pattern, the development of the aforesaid cursor dot and azimuth circle dot, and the development of D.C. positioning voltages to permit selective slewing of the cursor and circle. In this regard, oscillator 57 fulfills a major role, being a stable Source of sinusoidal voltage having a frequency of 1000 c.p.s. or thereabouts. The voltage of oscillator 57 is applied across circle time potentiometer R13 and cascode resolver driver 66. Timing motor and differential unit 15 is shown to be mechanically ganged with cursor and circle time potentiometcrs, R13 and R14, which rotate, each producing a voltage proportional to time. A time modifier hand control 19 is provided to augment the rotation of the potentiometers relative to their respective wipers. A time dial 18 is also mechanically connected as denoted and indicates the time remaining for interception, or as more familiarly termed the timeto-go. The AC. voltage of potentiometer R13 is applied to circle speed potentiometer and cathode follower 58, wherein a magnitude of AC. signal voltage is resolved which is proportional to the product of the interceptor speed and time. Manual interceptor speed control 72 is set in accordance with the predetermined speed of the interceptor. The phase shift network and push-pull amplifier S9 transforms the applied sinusoidal signal into two discrete voltages 90 electrical degrees apart to facilitate the generation of a circular pattern. These A.C. components thereupon undergo push-pull amplification and are supplied to an A.C. and DC. component mixer 61, which functions to superimpose these components onto a DC. positioning voltage developed in circle positioning circuit 63. A joystick control 62 is mechanically linked by means of a conventional gear arrangement to effect movement of a plurality of potentiometers which develop positioning voltages having magnitudes and polarities which correspond to displacement of the joystick from its neutral position. Thus, at the lower contacts abc-d of relay K301 are presented A.C. components in requisite phase relation for generation of a circular pattern, and which are each superimposed on a DC. level, the magnitude and polarity of which are a function of the position of the joystick. In a comparable manner, the purpose of cursor positioning circuit 65 is to establish a quadrantal number of discrete DC. voltage levels which correspond to the position of joystick 64, thereby providing control over the location of the cursor on the face of the CRT.

Cursor time potentiometer R14 is supplied a DC. voltage, a fractionated amount thereof being continuously apportioned according to time. Cursor speed potentiometer and cathode follower 73 accepts the fractionated voltage and produces in the output circuit thereof a low impedance DC. voltage of exceptionally linear characteristics that is proportional to distance. Cursor speed control 74 which modifies the relative position between wiper and the resistive portion of the potentiometer proper is manipulated under control of the operator to assure coincidence of the cursor dot S with the target video as displayed on the screen of the PPI scope. A DC. amplifier 75 performs amplification of the voltage level in addition to providing a measure of isolation. A two position selector switch 76, illustrated in FIG. 3b, is arranged to accop! the output of DC. amplifier 75 in the computer position. In the alternate radar position, switch 76 affords a convenient input from the range delay circuits of the radar, and in this respect, its specific application will be more apparent in later discussion in connection with the detailed schematic drawings of the instant invention. Assuming switch 76 to be in the computer position, the phantastron cathode follower 77 and phantastron 78 combinatively provide a bistable voltage having a duration in one of its stable states which is directly proportional to the magnitude of the voltage level of DC. amplifier 75. This bistable voltage waveform is applied to a delayed trigger amplifier 79, whose output is a positive differentiated spike voltage of approximately 35 volts, selectively delayed an amount proportional to the DC output level of amplifier 75. This spike waveform is applied to blocking oscillator 81, shown in FIG. 3c, which responds thereto and produces a low impedance strobe or cursor dot voltage which is made available at the lower paralleled a-b contacts of relay K302. Actuation of K302 positioning its armature in the down position permits the strobe voltage to be inserted into the video channel at strobe switch 53 in FIG. 30 upon appropriate gating. Thus, a strobe or cursor dot is provided in accordance with inventive concepts of the instant intercept computer.

In contrast to the specific use of essentially a DC. signal for generating the cursor dot voltage, the structure for effecting an azimuth circle dot relies upon the sinusoidal voltage of oscillator 57. A cascode resolver driver 66, illustrated in FIG. 3a, obtains its signal excitation from this source as previously denoted. The purpose of resolver driver 66 in addition to providing isolation is to present an exceedingly low impedance 1000 cycle signal to circle azimuth control resolver 68. The latter element resolves the applied input signal into two right angle voltage components as a function of the angular displacement of manual intercept heading control 67, which is mechanically linked in common with intercept heading indicator 82 and a rotor winding, not illustrated in FIG. 3a. Phase shift mixer 69 comprising a resistive-capacitive network recombines the components in a manner to efifect phase shift of the AC. signal in direct proportion to the angular rotation of control 67. The resultant sinusoidal signal emerging from mixer 69 may have its phase retarded -360 with respect to the output of oscillator 57 and is thence applied to azimuth dot generator 71, which comprises a two stage clipper amplifier, an RC shaping network, and a blocking oscillator. Collectively the elements making up azimuth dot generator 71 convert the phase shifted signal into a strobe voltage, which is made available at the upper paralleled ab contacts of relay K302. The armature of this relay in its up position, as shown, provides continuity to the video channel for insertion of the strobe voltage at strobe switch 53. It is of course manifested on the screen of the PPI scope as the azimuth circle dot D, depicted in FIG. 1.

The slewing potentials which locate the start of either the cursor or circle trace on the PPI screen are shown in FIG. 3a and FIG. 3b to be selectively applied to the respective N-S and E-W inputs of slewing cathode followers 83, 84, and 86 through the contacts of relay K301. The respective outputs of these stages are thence directly applied to PPI and cursor gated clamp stages 41, 42, 43 and 44, which upon receipt of appropriate gating signals permit the deflection amplifiers of the instant computer to be clamped at the level of the slewing potential. Provision is made for clamping the normal radar PPI sweeps at a zero level potential by grounding the respective inputs of stages 41 and 43, as indicated. Thus, the clamping means set forth assure that the various traces start from the same point of reference on the PPI screen. The gating signals for the PPI and cursor gated clamp stages are obtained from the PPI and cursor driver cathode followers 55 and 56. The latter stages are selectively fed in time shared relation by PH and cursor gate amplifier 87, which has dual output gating pulses. one of which is directly coupled to cathode follower 55. The other output is supplied to cathode follower 56 through contact c of K302 when the armature is disposed downwardly, its normal disposition in cursor application. When disposed upwardly, the normal disposition for circular pattern presentation, contact 0 supplies a cutoff bias to stage 56. In a similar manner, contact d of K302 in the downward position is effective to lower the cutoff bias on cursor switch stages 36 and 37, rendering these tubes conductive during cursor application. The precise functional significance of the gating and conditioning potentials associated with switch contacts c and d of relay K302 will be better appreciated in subsequent reference to the timing charts of FIG. 5 in connection with discussion of the system operation.

Hence, the structure as denoted in the functional block diagram of the inventive intercept computer provides for a plurality of integrated functions to be performed. Electrical circuit provisions have been set forth for performing the multiplication process, consonant with the generation of a coordinated time shared presentation consisting of cursor and circle traces integrated with normal radar PPI sweeps. In addition, cursor and azimuth circle dot voltages are developed and superposed onto the cursor and circle, respectively, for application in accordance with inventive concepts. The composite display as viewed on the PPI screen is coherently intensified by appropriate brilliance and blanking circuits.

Referring now to the views of FIGS. 4a through 411, there is illustrated in a composite showing of these views a detailed electrical diagram of the instant invention. It is to be noted that the alphabetical designations bordering these views are to be understood as points of common connection between the various sheets of the drawing, provided to facilitate relative orientation of the several sheets comprising FIG. 4.

Apropos of the matter of coordinating the views, it is deemed appropriate to consider first the sweep generating means of the instant invention with relation to the schematic showing thereof in FIG. 4g. While the sweep generating means illustrated therein is of a type existing in the indicator portion of the radar set incorporating the instant invention, it has been necessarily modified to facilitate time shared PPI display in accordance with inventive concepts. FIG. 4g, therefore, is a schematic illustration of a modified form of sweep generating means, and, it is to be further noted that the numerical underlined designa- 

10. IN AN EARLY WARNING RADAR SYSTEM HAVING A PLAN POSITION INDICATOR INCLUDING A ROTATING RADIAL SWEEP PRESENTATION, AN INTERCEPT COMPUTER FOR COMPUTING A TARGET INTERCEPT POINT BY EXTRAPOLATING PRESENT TARGET AND INTERCEPTOR POSITIONS TO RESPECTIVE FUTURE POSITIONS THEREOF COMPRISING, VIDEO CHANNEL MEANS FOR SUPPLYING TARGET AND INTERCEPTOR VIDEO TO SAID INDICATOR, MEANS INCLUDING A FIRST TIME POTENTIOMETER FOR GENERATING A SLEWABLE CIRCULAR PATTERN OF RADIUS PROPORTIONAL TO THE PRODUCT OF A PREDETERMINED INTERCEPTOR SPEED AND A DETERMINABLE FUTURE TIME INTERVAL, MEANS FOR PRODUCING A SLEWABLE CURSOR SWEEP ROTATIVELY CONTROLLABLE ABOUT THE ORIGIN THEREOF TO COINCIDE WITH TARGET DIRECTION, MEANS INCLUDING A SECOND TIME POTENTIOMETER FOR PRODUCING A CURSOR STROBE POTENTIAL HAVING A SELECTIVELY CONTROLLABLE DELAY FOR OBTAINING SYNCHRONISM OF A CURSOR DOT WITH THE SPEED OF THE TARGET VIDEO, A TIMING MECHANISM OPERABLY COUPLED WITH SAID FIRST AND SECOND TIME POTENTIOMETERS TO DEVELOP RESPECTIVE ELECTRICAL OUTPUTS PROPORTIONAL TO TIME, MANUAL MEANS OPERABLE TO MODIFY THE ELECTRICAL OUTPUTS OF SAID FIRST AND SECOND TIME POTENTIOMETERS FOR ADVANCING SAID CURSOR DOT ALONG THE CURSOR SWEEP CONCURRENTLY WITH A PROPORTIONATE INCREASE IN THE RADIUS OF THE CIRCULAR PATTERN, MEANS FOR GENERATING A CIRCLE STROBE POTENTIAL HAVING A SELECTIVELY CONTROLLABLE DELAY TO EFFECT DISPLAY OF A CIRCLE AZIMUTH DOT MOVABLE ABOUT THE PERIPHERY OF THE CIRCULAR PATTERN TO PROVIDE TARGET INTERCEPT HEADING, AND MEANS FOR PRODUCING A TIME SHARED PRESENTATION OF SAID RADIAL SWEEP, CURSOR SWEEP, AND CIRCULAR PATTERN TOGETHER WITH COHERENT INSER- 